This application relates to wireless communication devices, systems and techniques.
Various wireless communication systems use electromagnetic waves to communicate with wireless communication devices (e.g., mobile wireless phones) located within cells of coverage areas of the systems. A radio spectral range or band designated or allocated for a wireless communication service or a particular class of wireless services may be divided into different radio carrier frequencies for generating different communication frequency channels. Such systems use base stations spatially distributed to provide radio coverage in a geographic service area which is divided into cells. In such a cellular deployment, each base station (BS) is conceptually located at the center of a respective cell to provide radio coverage for that cell and transmits information to a wireless subscriber station (SS) such as a mobile SS (MSS) via BS-generated downlink (DL) radio signals. A subscriber station at a particular cell transmits information to its serving base station for that particular cell via uplink (UL) radio signals. The base stations may be designed to include directional antennas to further divide each cell into different sectors where each antenna covers one sector. This sectorization of a cell increases the communication capacity.
The capacity of wireless communications may also be increased by segmenting the communication bandwidth into different bandwidth segments for different cells and sectors. For example, in the frequency domain, Orthogonal Frequency Division Multiplexing (OFDM) and Orthogonal Frequency Division Multiple Access (OFDMA) physical layers in a cellular deployment can be used to divide the subcarriers of the uplink and downlink transmission into various orthogonal segments where each segment include multiple subcarriers. One or multiple segments can be grouped and designated for use in a sector of a cell. Because only a portion of the available subcarriers is used for communication in a sector, this use of the subcarriers is a partial use of the subchannels (PUSC). As an example, a PUSC permutation scheme is described the Partial Usage of Subchannels (PUSC) in the OFDMA physical layer (PHY) or Subchannelization in OFDM PHY defined in the IEEE 802.16-2004 and IEEE 802.16e-2005 standard documents.
FIG. 1A illustrates an OFDMA implementation of the bandwidth segmentation based on the PUSC. All available OFDMA subcarriers, all of which are used in a full use of subchannels (FUSC) scheme, are divided into different clusters or groups, e.g., groups 0 through 5. Assuming there are three segments, each segment includes one or multiple clusters of subcarriers or frequency tones but uses only part of the all available subcarriers or the total bandwidth provided in the system. This PUSC in sectors of a cell can reduce the interference. IEEE 802.16e PUSC or Subchannelization permutation scheme allows cellular deployment with a high degree of reduction in the interference at cell edges with adjacent cells and in intra-cell segment overlap areas. FIG. 1B shows a 3-sector cell based on the above signal bandwidth segment. In the cell, three antennas Ant-0, Ant-1 and Ant-2 that transmit signals at the three different frequency segments 0, 1 and 2, respectively, are used to transmit in the three sectors 0, 1 and 2, respectively. FIG. 1C further shows a 6-sector cell where 6 antennas at 6 different frequency segments are used to transmit into the 6 different sectors. Each sector of a cell can have its own set of subchannels in its frequency segment.
One technical difficulty in wireless communications is the signal fading during radio transmission from a transmitter to a receiver. Such signal fading can cause errors in reception of the signals and even lead to loss of data or failure of a communication link. One common cause for such fading is the notorious multipath fading where a signal reaches a receiver in two or more different signals paths, e.g., a direct signal and a delayed reflection from one or more objects. The fading of the signal at the receiver may be caused by, e.g., the interference of such different signals originated from the same signal from the transmitter. Transmit diversity techniques may be used to mitigate signal fading where two transmitter antennas may be used in a transmitter, e.g., the base station, to implement transmission diversity based on the space-time coding (STC). S. Alamouti described such a system to achieve a diversity of 2M using a receiver with M receiver antennas in “A simple transmit diversity technique for wireless communications” in IEEE Journal on Select Areas in Communications, Vol. 16, No. 8 (October 1998). A STC system can be implemented in various Multiple-Input Multiple-Output (MIMO) configurations.
FIG. 2 shows one example of a 3-sector cell that uses 2 transmitter antennas in each sector to provide the transmit diversity. The two antennas in the same sector transmit at the same signal bandwidth segment, e.g., the frequency segment 0, 1 or 2 shown in FIG. 1A and transmit the same signal of information of data with different STC codes. For example, in Sector 0, the antennas Ant-0 and Ant-3 transmit in the same frequency segment 0 with different STC codes that are represented by Segment 0 and Segment 0′. Similar notations are used in sectors 1 and 2. This cell design is equivalent to overlap of two cells where one cell has antennas Ant-0, Ant-1 and Ant-2 for the sectors 0, 1 and 2, respectively, and a second cell has antennas Ant-3, Ant-4 and Ant-5 for the sectors 0, 1 and 2, respectively. Each sector from one cell exactly overlaps with a corresponding sector in another cell that transmits the same signal of data or information at the same frequency segment under a different STC code.